docs: def build_kernel(self):
          """Build the KNN kernel.
  
          Build a k nearest neighbors kernel, optionally with alpha decay.
          Must return a symmetric matrix
  
          Returns
          -------
          K : kernel matrix, shape=[n_samples, n_samples]
              symmetric matrix with ones down the diagonal
              with no non-negative entries.
          """
          K = self.build_kernel_to_data(self.data_nu, knn=self.knn + 1)
          return K